Recall…. Princess Escape A princess has been grounded by her father (a wizard) and kept inside the castle. Being a rather rebellious princess, she has emailed the local dragon taxi service. The dragon will fly to the princess and she will climb aboard the dragon to escape from the castle – to meet some friends at the village dance club.
Textual Storyboard (pseudo code) Do in order dragon takes off dragon flies to princess princess climbs on dragon's back dragon and princess escape knight shakes his arm (and sword) in protest
In our programs, we have been using… Classes In Alice, classes are predefined as 3D models Objects An object is an instance of a class. Class: Frog (Uppercase name) Objects: frog, frog1, frog2, frog3 (lowercase names)
We have also used… built-in (predefined) methods Examples: move, turn to face, say World.my first method Example: In the Dragon world, we wrote program code where a dragon took off (as the first step in our animation). All the program code was written in this one method, see next slide…
Larger Programs As you become more skilled in writing programs, you will find that programs quickly increase to many, many lines of code. Games and other "real world" software applications can have thousands, even millions of lines of code.
Potential Problem The program code just seemed to grow and grow. If we continue to write programs this way the programs will become longer and more difficult to read and think about.
Solution A solution is to organize the instructions into smaller tasks.
Stepwise Refinement The process of breaking a problem down into large tasks and then breaking each task down into simpler steps is called stepwise refinement. Once the storyboard is completed, we write a method for each task.
Stepwise refinement - 1 How can a dragon "take off"? Do together dragon moves up dragon flaps wings Do in order dragon takes off dragon flies to princess princess climbs on dragon's back dragon and princess escape knight shakes his arm (and sword) in protest
Stepwise refinement - 2 How can a dragon flap its wings? Do together dragon flap left wing dragon flap right wing Do together dragon moves up dragon flaps wings
Stepwise refinement - 3 How can a dragon flap its left wing? Do in order Do together dragon close left wing rolls right dragon far left wing rolls right Do together dragon close left wing rolls left dragon far left wing rolls left Do together dragon flap left wing dragon flap right wing
Demo Create the flapWings method Create a takeOff method, where the dragon moves up 2 meters and flaps its wings twice What changes will you need to make to the duration= parameter to get the animation working?
Summary Why do we want to write our own methods? saves time -- we can call the method again and again without reconstructing code reduces code size – we call the method rather than writing the instructions again and again allows us to "think at a higher level" can think surprise instead of “The alien moves up and says ‘Slithy toves?’ and then the robot's head turns around. " the technical term for "think at a higher level" is "abstraction"
Summary: Classes, Objects, & Methods Object-oriented programming uses classes, objects, and methods as basic programming components. These components help to organize a large program into small modules design and think about an intricate program find and remove errors (bugs)
Assignment Read Chapter 4, Section 3 Class-level Methods How to create them How to call them